Internet of Things (IoT) is quickly evolving within the telecommunications realm. Current 3rd Generation Partnership Project (3GPP)-based standards offer three different variants supporting IoT services: enhanced Machine-Type Communications (eMTC), Narrow Band IoT (NB-IoT), and Extended Coverage-Global System for Mobile Communication (EC-GSM). eMTC and NB-IoT have been designed using LTE as a baseline, with the main difference between the two being the minimum occupied bandwidth, where eMTC and NB-IoT use 1.4 MHz and 180 kHz minimum bandwidth respectively. Both NB-IoT and eMTC have been designed with an operator deployment of macro cells in mind. Certain use cases where outdoor macro evolved NodeBs (eNBs) would communicate with IoT devices deep inside buildings are targeted, which require standardized coverage enhancement mechanisms.
3GPP Long Term Evolution (LTE) Rel-12 defined a User Equipment (UE) power saving mode allowing long battery lifetime and a new UE category allowing reduced modem complexity. 3GPP Rel-13, further introduced the eMTC feature, with a new category (Cat-M) that further reduced UE cost while supporting coverage enhancement. The key element to enable cost reduction for a Cat-M UE is to introduce a reduced UE bandwidth of 1.4 MHz in the downlink and the uplink within any system bandwidth. See 3GPP TR 36.888, “Study on Provision of Low-Cost Machine-Type Communications (MTC) User Equipments based on LTE,” which is referred to herein as “TR 36.888.”
In LTE the system bandwidth can be up to 20 MHz, where this total bandwidth is divided into Physical Resource Blocks (PRBs) having a predetermined bandwidth, e.g., 180 kHz. Cat-M UEs with a reduced UE bandwidth of 1.4 MHz only receive a part of the total system bandwidth at a time, e.g., a part corresponding to up to 6 PRBs, where a group of 6 PRBs may be referred to as a “PRB group.”
In order to achieve the coverage targeted in LTE Rel-13 for low-complexity UEs and other UEs operating delay tolerant MTC applications, as disclosed in TR 36.888, time repetition techniques are used to allow energy accumulation of the received signals at the UE. For physical data channels, e.g., a Physical Downlink Shared CHannel (PDSCH), Physical Uplink Shared CHannel (PUSCH), etc., subframe bundling (a.k.a. Transmission Time Interval (TTI) bundling) may be used. When subframe bundling is applied, each Hybrid Automatic Repeat reQuest (HARQ) (re)transmission comprises a bundle of multiple subframes instead of just a single subframe. Repetition over multiple subframes is also applied to physical control channels.
Energy accumulation of the received signals involves several aspects. One of the main aspects involves accumulating energy for reference signals, e.g., by applying time-filters, in order to increase the quality of channel estimates used in the demodulation process. Another important aspect involves accumulating demodulated soft-bits across repeated transmissions.
European Telecommunications Standards Institute (ETSI) EN 300 328 provisions several adaptivity requirements for different operation modes. From the top level, equipment can be classified either as frequency hopping or non-frequency hopping, as well as adaptive or non-adaptive. Adaptive equipment is mandated to sense whether the channel is occupied in order to better coexist with other users of the channel. The improved coexistence may come from, e.g., Listen Before Talk (LBT) or Detect And Avoid (DAA) mechanisms. Non-frequency hopping equipment are subject to requirements on maximum Power Spectral Density (PSD) of 10 dBm/MHz, which limits the maximum output power for systems using narrower bandwidths. One commonality for any of the adaptive schemes is the consequence that the receiving node will be unaware of the result of the sensing, and thus needs to detect whether a signal is present. While such signal detection most likely would be feasible for devices operating in moderate to high Signal-to-Interference-plus-Noise Ratio (SINR) levels, such signal detection may be infeasible for very low SINR levels.
For systems using repetition schemes to achieve coverage extension, the received SINR of each individual transmission is very low. Through accumulation of multiple transmissions, the effective SINR increases. However, in cases where the accumulation includes both signal as well as noise, as could be the case when the transmitter uses adaptive mechanisms, the repetition techniques may fail. One way of avoiding this failure would be to attempt detection of each individual repetition, although as already mentioned this may not be feasible at the very low SINR levels targeted with these IoT standards.
An IoT standard for 2.4 GHz in Europe may therefore be best devised by categorizing its devices as non-adaptive frequency hopping. Requirements for non-adaptive frequency hopping include the following key parts, which are also shown in FIG. 1:                A maximum on-time of 5 ms (“ON”), which is required to be followed by a transmission gap.        A minimum duration of the transmission gap of 5 ms (“OFF”).        A maximum accumulated transmit time of 15 ms on one frequency (e.g., the three ON periods on Frequency 1 in FIG. 1), which is the maximum total transmission time a node may be allowed to use before moving or hopping to the next frequency (e.g., Frequency 2 in FIG. 1).Wireless communication devices configured to operate according to a frequency hopping pattern have unique synchronization challenges due to the ever changing transmission frequency. For example, some synchronization solutions are costly, both in terms of delay and power consumption, while other synchronization solutions do not provide sufficient accuracy or reliability. Synchronization for frequency hopping devices is an important issue, particularly given the repeated need for synchronization, e.g., after the device wakes up from a sleep mode. U.S. Patent Publication No. 2014/0301263, discloses selection of various alternative wake up procedures to address the timing resolution for UEs that operate using a discontinuous reception (DRX) mode that includes various sleep cycles. A UE selects a wake up procedure based on the length of the sleep cycle. The UE may use details of the sleep cycle, including a time offset or timing uncertainty associated with the sleep cycle, when selecting the wake-up procedure.There is a need for a solution that handles synchronization for devices configured to operate according to a frequency hopping pattern.        